1. Field of the Invention
The present invention relates to a board-to-board optical interconnection system for connecting respective boards by virtue of very high speed, super high density and huge capacity optical signals in such a system that a plurality of boards on which electronic parts such as very high speed, super high density and large capacity ATM (Asynchronous Transfer Mode) switch LSIs are mounted are assembled like a book shelf.
In addition, the present invention relates to a unit-to-unit(shelf-to-shelf) optical interconnection system for connecting respective boards in different units by virtue of the optical signals in such a system that a plurality of boards on which electronic parts such as very high speed, super high density and large capacity ATM switch LSIs are mounted are assembled like a book shelf respectively to be packaged in a plurality of units which are then arranged in a cabinet in a matrix fashion.
2. Description of the Prior Art
As shown in FIG. 1, N.times.N switches are arranged in L rows and M columns to construct a large capacity switching system. 1-1 is an N.times.N switch, and 1-2 is a wiring for connecting the N.times.N switches 1-1. In order to reduce cell loss probability, the N.times.N switches are also arranged in a multistage fashion. More particularly, as shown in FIGS. 2A and 2B, a plurality of boards 2-1 are assembled like a book shelf and then the boards 2-1 are mutually connected via board-to-board optical interconnections 2-2. In addition, in order to realize the large capacity switching system by virtue of book shelf type assembling, an arrangement as shown in FIG. 3 is needed. In other words, a plurality of units(shelves) 3-2, in each of which the boards 2-1 are assembled like the book shelf and then the boards 2-1 can be connected via board-to-board optical interconnections 2-2, are arranged in the multistage fashion. At this time, respective boards 2-1 between different units 3-2 can also be optically connected via unit-to-unit optical interconnections 3-3. 3-1 is a cabinet in which all the units 3-2 are housed. Schematic appearances of the cabinet and the units are shown in FIG. 4. 4-1 is a door, 4-2 are beams emitted from the unit 3-2 and input into the unit 3-2, and 4-3 is an electrical connector connected to an electric backplane.
Meanwhile, ATM switching board interconnection which have been practically used at present are made in principle of electric wirings. Performance of the interconnection is determined according to a pin density of the electrical connector and a transmission rate of the signal. Currently the pin density of the electrical connector is about 1/mm.sup.2. A power of about 1W is at present needed to transmit the electric signal over several centimeters at a transmission rate of 100 Mbit/s, and therefore heat radiation must be taken account. Besides, there has been the problem that EMC (ElectroMagnetic Compatibility) noises are generated when high speed signals travels through the boards. For this reason, it has been said that, if the boards are connected via the electric signals, the critical transmission rate is several 100 Mbit/s and the critical connector density is 1/Mm.sup.2. However, the transmission rate of the signal and the pin density of the connector tend to increase year by year, thereby approaching their critical values. In order to overcome such critical values, board-to-board optical interconnection has been given much attention in the art. As the optical interconnection between ATM switch boards or between the units, an optical interconnection module, in which a semiconductor laser array and a detector array are connected via an optical fiber array, has been developed and has already reached a commercially available stage (see J. Nishikido, S. Hino, S. Urushidani and K. Yamasaki; "Demonstration of Optically Interconnected Switching Network", GLOBECOM '93, pp. 1187-1191).
However, throughput of the optical interconnection module of fiber type is about several tens Gbit/s at most. Such throughput of the optical interconnection module is not sufficient for optical interconnection between future huge capacity ATM switch boards whose throughput is in the range from 1 Tbit/s to 10 Tbit/s. Therefore, a free-space optical interconnection, in which the transmitter and the detector can be directly connected without the optical fiber by rendering optical beams to travel in a free-space, has been studied as a promising candidate. The free-space optical interconnection has such various excellent advantages that no mutual interference exists between optical signals, super high density optical interconnection can be established, low skew can be achieved between optical signals, no electric noise is caused, lower optical coupling loss can be achieved, etc.
Various approaches have been proposed to implement the huge capacity ATM switch. An example of such approaches will be explained with reference to FIG. 5. As shown in an upper detailed view in FIG. 5, N.times.N subnetworks LSI 5-2 (including MCM packaging) are first prepared with the use of 2.times.2 switches 5-1 as basic unit switches and then mounted on the board. Then, these boards are arranged like a book shelf of M stage and L column and then connected via optical paths. An advantage of this packaging approach resides in that, once the basic N.times.N subnetwork switch boards are prepared, it is feasible to freely expand and reduce the capacity of the switch by arbitrarily changing the numbers of M stage and L column of the book shelf. If these boards are arranged like the book shelf, parallel and cross interconnections extending over plural boards are needed, as shown in FIG. 6A.
For contrast, there has been another approach wherein 2.times.2 basic unit switches 5-1 are cut out as vertical regions, as shown by a broken line a in FIG. 5, and then assembled on one sheet of the board. In this case, parallel and cross interconnections between neighboring boards are applied as interconnections, as shown in FIG. 6B. As one example of this configuration, there has been the digital regenerative optical switch (SEED) system 5 proposed by the Bell Laboratories or the EARS switch (16.times.16 switch, four stage arrangement) proposed by the NTT, both having been studied for the purpose of large capacity ATM switch. An advantage of these configurations is that they are suited for super high density optical interconnection since the chip-to-chip interconnection and the board-to-board interconnection are required only between neighboring chips and neighboring boards.
A number of reports on the free-space optical interconnection for connecting the neighboring boards have been delivered.
For instance, Hinton et al. with the Magill University have proposed the optical backplane in which free-space super-parallel optical interconnection and the optical digital regenerative switch (SEED) are employed as the backplane (T. Szymanski and H. S. Hilton, "An Architecture of a Terabit Free-Space Photonic Backplane", The International Conference on Optical Computing Technical Digest, OC'94, Edinburgh, Scotland, Aug. 22-25, (1944) WD2/221).
Further, in order to optically connect parallel processors, Sakano et al. with the NTT have accomplished the 20 Mbit/s neighboring board free-space optical interconnection system, though low density and low speed, in which four sheets of the boards on which 4.times.4 LEDs and detectors are mounted are arranged (T. Sakano, T. Matsumoto, and K. Noguchi; "Three-Dimensional Board-to-Board Free-Space Interconnections and Their Application to the Prototype Multiprocessor System: COSINE-III", Applied Optics, vol. 34, pp. 1815-1822, 1995).
Still further, as the high speed neighboring board free-space optical interconnection, D. Z. Tsang with the MIT has finished the optical interconnection with 20 channel at a transmission rate of 1 Gbit/s per channel by transmitting collimated optical beams from the semiconductor laser over a 24 cm distance with the use of micro positioner (D. Z. Tsang; "One-Gigabit per Second Free-Space Optical Interconnection", Applied Optics, vol. 29, pp. 2034-2037, 1990).
In the meanwhile, as the board-to-board optical interconnection, there have been the optical interconnection in which board-to-board bus connection can be realized by means of D-fibers (P. Healey; "Chapter 7 Multidimensional Switching Systems in Photonics in Switching, Vol. II", Edited by J. E. Midwinter, Pressed by Academic Press (London)), the optical interconnection in which holograms are employed on the backplane (P. C. Kim; "An Optical Holographic Backplane Interconnect System", J. Lightwave. Tech. Vol. 9, pp. 1650-1656, 1991). In addition to the above, Mikazuki et al. with the NTT has connected the boards by optical fibers via optical couplers and distributed the clock of 1 Gbit/s for the purpose of board-to-board optical bus interconnection (K. Itoh, R. Konno, Y. Katagiri, and T. Mikazuki; "Data Transmission Performance of an Optical Backboard Bus", Proc. of Japan IEMT, pp. 268-271, 1995).
However, in the optical interconnection above described in the prior art, there has arisen the problem that intermediate amplifiers have to be provided in the optical interconnection using the D-fibers since considerable loss is caused therein.
Besides, in the board-to-board optical interconnection using the stationary holographic backplane, there have been such problems that unnecessary higher order lights such as zero order, -first order, .+-.second order, etc. lights other than the desired light are generated to thereby increase crosstalk, and it entails too enormous cost to fabricate a large hologram covering the overall backplane, and further it is difficult to achieve the optical beam alignment such that the wavelength of the transmitter can be controlled precisely so as to establish desired optical interconnection.
In the board-to-board optical interconnection in which the optical fibers are placed via the optical couplers, there has been such a problem that its throughput has been restricted to several tens Gbit/s at most.
Moreover, in the prior art, there has been such a problem that merely the electric interconnection using the electric backplane is presented as the unit-to-unit interconnection and its throughput is very low. In addition, the optical interconnection in which the units are connected by the optical fibers has been studied, but its throughput is low, e.g., 10 Gbit/s at most in the existing state.